This invention relates to a flow divider, and more particularly relates to a novel flow divider for equalizing the distribution of fluid fuel to the nozzles of a plurality of combustion cans of a gas turbine arrangement.
Flow dividers for ensuring a continuous equal distribution of a fluid to several different outlets are well known. Conventionally, a plurality of spaced sets of metering gears is provided and a common inlet conduit is connected to each of the metering gear sets. The output fluid from each of the metering gear sets should then be always balanced regardless of the hydraulic impedance in any of the outlet channels. Configurations of this general type are shown, for example, in U.S. Pat. Nos. 2,291,578 to Johnson, 3,601,139 to Kontranowski, 3,495,610 to Van Aken, Jr., 2,343,912 to Lauck and 796,724 to Hewitt. Flow dividers of this general type are presently employed in gas turbines for distributing fuel oil between the different combustion can nozzles of the gas turbine.
The provision of equal quantities of fuel to the different combustion cans must be maintained under at least two critical conditions. The first is during turbine start-up when the flow divider must compensate for the differences in elevation between the combustion cans and to ensure that equal quantities of fuel go to each combustor chamber. The second condition is during normal operation when the different nozzles accumulate different thickness coke deposits. The flow dividers must continue to supply similar quantities of fuel to the various combustion chambers even though the back-pressure drops in the chambers vary due to unequal coking of the various fuel nozzles of the turbine.
The present fuel divider designs which are now in use in gas turbines use the fuel oil being divided as the lubricating fluid for the bearings of the metering gear shafts. This lubrication is satisfactory when using clean fuel oil. However, when running with certain crude oils and residuals, the metering shaft lubrication has been unsatisfactory and has caused excessive wear and failure of the bearings which were used. The bearings which were previously used consisted of numerous relatively small needle bearings; 20 needle bearings to 40 needle bearings in different designs which were used. The bearings which were used were also subject to corrosion but this problem can be solved substantially through the use of appropriate corrosion-resistant alloys.
The gas turbine flow dividers which are presently in use are also of a design which requires relatively complex assembly and disassembly when the device is to be inspected or maintained. Furthermore, entirely different structures and inventories of parts are required when machines are to have a different number of outlet chambers. Moreover, the designs which are presently used must be tailor-made for different shaft rotation speeds and the designs now used do not lend themselves to manufacturing standardization.
Finally, the designs presently in use are not tolerant of thermal excursions in the fluid being metered. When such excursions occur, the various components of the flow divider increase or decrease in length as the case may be to a degree that is proportional to the temperature rise of the individual part and its coefficient of thermal expansion. When such changes in length or size occur, the clearances provided frequently are closed down, and binding and seizing of the flow divider may occur. In gas turbine operation, such binding or seizing of the flow divider can result in the loss of fuel to the combustion system, and the consequent tripping out of the gas turbine from loss of flame. It should be noted that thermal transients are common in gas turbines operating on crude or residual fuels, as characteristically these fuels have to be heated to temperatures as high as 200.degree. F. in order to achieve proper viscosity characteristics for combustion, and to ensure that all the wax particles in the fuel have been melted. At the same time, the gas turbines start up and shut down on distillate fuel, which is characteristically at a temperature of 50.degree. F. It is during the transfer from the distillate fuel to the crude or residual fuel that the temperature transient on the order of 150.degree. F. is experienced. This thermal transient takes place in a matter of a few seconds.